The arrangement of a typical prior art thermal printhead 10 is shown in FIGS. 8-11. Reference numeral 11 indicates a head substrate. The substrate, which is made of an insulating material such as alumina-ceramic, has an upper surface provided with a heating element 12 and a plurality of drive ICs 13 for driving the heating element 12. When the thermal printhead is a thick film-type, the heating element 12 is made by a thick film printing method and configured into a narrow strip extending along a side edge of the substrate. FIG. 11 shows a common electrode 14 having comb-like teeth 14a extending under the heating element 12 and individual electrodes 15 arranged like comb-like teeth. Each individual electrode 15 extends toward the other side edge of the head substrate to be wire-bonded to an output pad of a drive IC 13. Each drive IC 13 includes power pads and signal pads which are wire-bonded to a predetermined wiring pattern formed on the substrate.
When a selected individual electrode 15 is turned on by a corresponding drive IC 13, an electric current is passed across a portion (shaded in FIG. 11) defined by a pair of comb-like teeth 14a of the common electrode 14 which sandwich the selected individual electrode therebetween, and heat is generated at the portion. In this way, the respective portions defined between the comb-like teeth 14a of the common electrode 14 function as heating dots 17. Each of the comb-like teeth 14a of the common electrode 14 is rendered to have a very small width. The teeth are spaced from each other by 125 .mu.m when a desired printing density is 200 dpi for example. This also applies to the individual electrodes 15. The minute wiring patterns including the common electrode and individual electrodes are formed by etching a conductive layer made of e.g. gold applied over the substrate.
To produce a thermal printhead having the above-mentioned printing density of 200 dpi and capable of printing on A4-size printing paper, 1728 heating dots 17 are arranged in line on the head substrate. When the drive IC 13 has 64-bit output pads, 27 drive ICs are mounted on the head substrate. The head substrate 11 is also provided with a thermistor 18 as a temperature sensor for monitoring the temperature of the heating element 12. Generally, the thermistor 18 is disposed at a longitudinally central portion of the head substrate 11 and between two adjacent drive ICs 13 for convenience of arrangement of the wiring pattern. The drive ICs and wire-bonded portions are enclosed by a protective coating 19 made of an epoxy resin for example.
A heat sink 20 is made of a material providing good heat dissipation such as aluminum. The head substrate 11 is attached to the heat sink 20 via an acrylic resin adhesive 21 for example.
The head substrate 11 is made of a fragile insulating plate. However, the strength of the thermal printhead as a whole is properly maintained by mounting the head substrate 11 on the heat sink 20 which has great mechanical strength. Further, such an arrangement improves the printing quality, since heat generated at the heating element 12 during operation of the printhead is conducted to the heat sink.
The printing operation by the above thermal printhead is performed for each line. For this, output pads corresponding to selected bits are turned on for a predetermined time, based on the 1728-bit printing data serially input in shift registers of the drive ICs 13.
For high-speed printing, a printing period (the interval between the starting point of a printing operation and the starting point of the next printing operation) should be shortened. Also, it is necessary to control driving power supplied to the heating element 13 by monitoring the temperature of the heating element, so that so-called trailing phenomenon and fainting phenomenon are avoided. Specifically, the actuating time for heat generation (the width of a printing pulse) is adjusted within a printing period by monitoring heat generated by the heating element 12 with the use of the thermistor 18. For instance, when so-called solid printing is continuously performed, it is necessary to properly shorten the width of the printing pulse, thereby preventing the total amount of heat generated at the heating element from becoming unduly large. In this way, it is possible to avoid a trailing phenomenon at the end of the solid printing area. Conversely, at an initial stage of actuation of the thermal printhead, the width of the printing pulse is caused to increase for the purpose of supplying a large amount of driving power to the heating element. This is because the thermal printhead must start at room temperature.
Regarding the conventional thermal printhead, the heat sink 20 also extends under the location of the thermistor 18, as shown in FIG. 9. In such a case, the heat generated by the heating element 12 reaches the thermistor 18 via the head substrate 11 as a first path and via the heat sink 20 as a second path. The head substrate 11 and the heat sink 20 have different thermal conductivities and the lengths of the paths to the thermistor 18 are different. As a result, the heat detected by the thermistor 18 is a combination of heats conducted along the respective paths whose thermal conduction and conduction length are different. In general, variation of the heat conducted through the head substrate 11 made of a relatively thin alumina-ceramic plate is detected with a relatively good response. On the other hand, variation of the heat conducted via the heat sink 20, which is made of an aluminum plate having a thickness of more than a certain value, is detected with a relatively improper response. Therefore, the relation between the time and the temperature of the heat which is conducted via different paths and detected by the thermistor 18 does not necessarily reflect the temperature variation occurring at the heating element. Thus, the printing pulse control based on such detection may fail to provide an optimum control of the printing power for the heating element.
For solving the above problems, the adhesive 21 may be replaced for another one having a smaller thermal conductivity, so that a less amount of heat is conducted into the heat sink. Thus, the interference by the heat conducted via the heat sink may be reduced. However, this solution will give rise to another problem described below.
To properly control the width of a printing pulse for high-speed printing for example, the heat generated by the heating element 12 should be effectively conducted into the heat sink 20. This is because insufficient heat conduction into the heat sink 20 will cause an unduly rapid increase in temperature of the heating element 12 when, for example, plural lines are sequentially printed. To deal with this problem, the width of a printing pulse may be shortened. However, this solution will overload the control unit (CPU) performing the control. Such a situation may be properly dealt with by utilizing a control unit (CPU) which is capable of performing a remarkably high speed processing. However, the cost for it is unduly high, and therefore such a unit may not be readily adopted.
The replacement of the adhesive 21 gives rise to the problem described above. Besides, with the above replacement, the temperature detecting response of the temperature sensor can only be varied within a small range. As another way to render the temperature sensor to detect a temperature variation which reflects the temperature variation of the heating element more properly, it is possible to arrange the thermistor very close to the heating element. However, this solution necessitates modification of basic arrangements of the head substrate, and therefore is disadvantageous in terms of costs for manufacturers of thermal printheads.